What was the hammer used for? :}
It should be interesting to see how well it performs.
I'm not a mechanical Engineer but I've worked on a number of such systems. I can't see clearly but this seems to be an overly constrained system (constraining a rigid body in more than 6 ways). Basically if everything is not perfect the axis will bind. Nothing is ever perfect. The rails and screw will not be perfectly straight or parallel. The better and stiffer the system is built the more this becomes an issue. Normally flexures are used so that one of the rails or nut is allowed some amount of controlled "float" or compliance in one direction but still be highly rigid in the other direction where it is needed. Sometimes by luck there is enough compliance in the system so it works ok. Have you considered this?